I. Field
The following description relates generally to wireless communications, and more particularly to searching and/or tracking of neighboring cells for handover or other applications, such as, location inference and/or cooperative transmission from base stations.
II. Background
Wireless communication systems are widely deployed to provide various types of communication; for instance, voice and/or data can be provided via such wireless communication systems. A typical wireless communication system, or network, can provide multiple users access to one or more shared resources (e.g., bandwidth, transmit power, . . . ). For instance, a system can use a variety of multiple access techniques such as Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), Code Division Multiplexing (CDM), Orthogonal Frequency Division Multiplexing (OFDM), 3GPP Long Term Evolution (LTE) systems, and others.
Generally, wireless multiple-access communication systems can simultaneously support communication for multiple access terminals. Each access terminal can communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from base stations to access terminals, and the reverse link (or uplink) refers to the communication link from access terminals to base stations. This communication link can be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
MIMO systems commonly employ multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas can be decomposed into NS independent channels, which can be referred to as spatial channels, where NS≦{NT, NR}. Each of the NS independent channels corresponds to a dimension. Moreover, MIMO systems can provide improved performance (e.g., increased spectral efficiency, higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
MIMO systems can support various duplexing techniques to divide forward and reverse link communications over a common physical medium. For instance, frequency division duplex (FDD) systems can utilize disparate frequency regions for forward and reverse link communications. Further, in time division duplex (TDD) systems, forward and reverse link communications can employ a common frequency region so that the reciprocity principle allows estimation of the forward link channel from reverse link channel.
Wireless communication systems oftentimes employ one or more base stations that provide a coverage area. A typical base station can transmit multiple data streams for broadcast, multicast and/or unicast services, wherein a data stream may be a stream of data that can be of independent reception interest to an access terminal. An access terminal within the coverage area of such base station can be employed to receive one, more than one, or all the data streams carried by the composite stream. Likewise, an access terminal can transmit data to the base station or another access terminal.
Access terminals or user equipment (UE) wishing to access a long term evolution (LTE) cell typically must first undertake a cell search procedure. This can consist of a series of synchronization stages by which the UE determines time and frequency parameters necessary to modulate downlink and transmit uplink signals with the correct timing. At this stage, the access terminal or UE also acquires some critical system parameters.
Three major synchronization requirements can be identified in the LTE™ system: the first is symbol timing acquisition, by which the correct symbol start position is determined, for example, to set the Fast Fourier Transform (FFT) window position; the second is carrier frequency synchronization, which is typically required to reduce or eliminate the effect of frequency errors arising from a mismatch of local oscillators between transmitter and receiver, as well as the Doppler shift that can be caused by any UE motion; thirdly, sampling clock synchronization can also be necessary.
Two relevant cell search procedures exist in LTE™: initial synchronization whereby the access terminal or UE detects a LTE™ cell and decodes all the information required to register it—typically performed for example, when the UE is switched on, or when the UE has lost connection to its serving cell; and new cell identification, generally performed when the UE is already connected to an LTE™ cell and is in the process of detecting a new neighbor cell.
In both scenarios, the synchronization procedure can utilize physical signals broadcast in each cell: the primary synchronization channel (PSC) and the secondary synchronization channel (SSC). Detection of these two signals not only enables time and frequency synchronization, but also provides the access terminal or UE with the physical layer identity of the serving cell and the cyclic prefix (CP) length, and further informs the access terminal or UE whether the cell employs frequency division duplex (FDD) or time division duplex (TDD).